A related study at Karolinska is said to have resulted in people perceiving that their physical selves were located not in their own bodies, but in those of other test subjects.

It is possible to create an illusion of owning three arms, under controlled conditions in a laboratory. The experiment involves the participant sitting at a table and having a realistic prosthetic arm placed next to their right arm. The subject then sees her two real arms and the extra prosthetic arm, made out of rubber. To produce the feeling of owning the rubber arm, the scientist touches the subject’s right hand and the rubber hand with two small brushes at corresponding location – synchronizing the strokes as perfectly as possible.

“What happens then is that a conflict arises in the brain concerning which of the right hands belongs to the participant’s body”, says Arvid Guterstam, one of the scientists behind the study. “What one could expect is that only one of the hands is experienced as one s own, presumably the real arm. But what we found, surprisingly, is that the brain solves this conflict by accepting both right hands as part of the body image, and the subjects experience having an extra third arm.”

Could it be possible that, in the not-so-distant future, we will be able to reshape the human body so as to have extra limbs? A third arm helping us out with the weekly shopping in the local grocery store, or an extra artificial limb assisting a paralysed person? Here we report a perceptual illusion in which a rubber right hand, placed beside the real hand in full view of the participant, is perceived as a supernumerary limb belonging to the participant’s own body. This effect was supported by questionnaire data in conjunction with physiological evidence obtained from skin conductance responses when physically threatening either the rubber hand or the real one. In four well-controlled experiments, we demonstrate the minimal required conditions for the elicitation of this “supernumerary hand illusion”. In the fifth, and final experiment, we show that the illusion reported here is qualitatively different from the traditional rubber hand illusion as it is characterised by less disownership of the real hand and a stronger feeling of having two right hands. These results suggest that the artificial hand ‘borrows’ some of the multisensory processes that represent the real hand, leading to duplication of touch and ownership of two right arms. This work represents a major advance because it challenges the traditional view of the gross morphology of the human body as a fundamental constraint on what we can come to experience as our physical self, by showing that the body representation can easily be updated to incorporate an additional limb.

Out-of-body experiences (OBEs) have fascinated people for millennia – their existence has raised fundamental questions about the relationship between human consciousness and the body, and has been much discussed in theology, philosophy and psychology. An OBE occurs when a person who is awake sees his or her own body from a location outside the physical body. OBEs have been reported in clinical conditions where brain function is compromised, such as stroke, epilepsy and drug abuse, or in association with traumatic experiences such as car accidents. Around one in ten people claim to have had an OBE at some point in their lives.

However, the neuroscience behind this phenomenon remains obscure. Henrik Ehrsson, a 38-year-old cognitive neuroscientist at the Karolinska Institutew1 in Stockholm, Sweden, investigates how the brain represents the self, and has induced the phenomenon of OBE in healthy volunteers for the first time. He recreated an illusion in which individuals look at themselves from outside their physical bodies.

The volunteers wear goggles with a video screen for each eye. Each screen is fed with images from a separate camera behind the volunteer and, because the two images are combined by the brain into a single image, the individual sees a stereoscopic (3D) image of his or her own back. We then move a plastic rod towards a location just below the cameras while the volunteer’s chest is simultaneously touched in the corresponding position. The participants reported feeling that they are located back where the cameras are placed, watching a body that belongs to someone else.

Inducing an out-of-body experience Image courtesy of Henrik Ehrsson

To test the illusion further and provide objective evidence, I then performed an additional experiment to measure the volunteers’ physiological response – specifically the level of perspiration on the skin – in a scenario in which they feel that their illusory body is threatened. Using a hammer, we ‘attacked’ a point below the cameras – a point where, according to the illusion, the volunteer’s body was located. The volunteer’s bodily response strongly indicated that he or she thought the threat was real: at the moment when the hammer ‘hit’ the illusory body, the volunteer sweated more. This demonstrates that the experience of being localised within the physical body can be determined by the visual perspective in conjunction with correlated multisensory information from the body.

Body swapping with a mannequin Image courtesy of Staffan Larsson

Then you went further, successfully creating the illusion of body swapping by making volunteers perceive the bodies of mannequins and other people as their own, putting a male in a female body, a young person in an old body, a white person in a black body, and vice versa.

Yes, we presented evidence that the brain, when tricked by optical and sensory illusions, can quickly adopt another human form as its own, no matter how different it is. We designed two experiments. In the first one, the researchers fitted the head of a mannequin with two cameras connected to two small screens placed in front of the volunteer’s eyes, so that the volunteer could see what the mannequin ‘saw’.

When the mannequin’s camera eyes and the volunteer’s head, complete with the camera goggles, were directed downwards, the volunteer saw the dummy’s body where he or she would normally have seen his or her own body. By simultaneously touching the stomachs of both the volunteer and the mannequin, we could create the illusion of body swapping.

The volunteer could then see that the mannequin’s stomach was being touched while feeling (but not seeing) a similar sensation on his or her own stomach. Thus, the volunteer developed a powerful sensation that the mannequin’s body was his or her own.

In the second experiment, we mounted the camera onto another person’s head. When this person and the volunteer turned towards each other to shake hands, the volunteer perceived the camera-wearer’s body as his or her own. The volunteers saw themselves shaking hands, experiencing it as though they were another person. The sensory impression from the handshake was perceived as though coming from the new body, rather than the volunteer’s own.

The strength of the illusion was proved when the volunteers exhibited stress reactions after a knife was held to the camera-wearer’s arm but not when it was held to their own. However, the volunteers could not fool themselves into identifying with a non-humanoid object, such as a chair or a large block.